RE: TTML2/TTML1 Backwards compatibility analysis

>> In this exercise I was concerned with how a TTML1 document (not containing any TTML2 feature) would be rendered by a TTML2 presentation processor…

Good.  I think it would be helpful to say that in the document.

               Mike

From: Cyril Concolato [mailto:cconcolato@netflix.com]
Sent: Wednesday, October 18, 2017 1:58 PM
To: Michael Dolan <mike@dolan.tv>
Cc: Nigel Megitt <nigel.megitt@bbc.co.uk>; public-tt@w3.org
Subject: Re: TTML2/TTML1 Backwards compatibility analysis

Thank you Mike for the feedback. See my comments inline.

On Wed, Oct 18, 2017 at 8:52 AM, Michael Dolan <mike@dolan.tv<mailto:mike@dolan.tv>> wrote:
This is definitely a good exercise – thanks Cyril.  A few comments:

  1.  The analysis is not clear about whether it covers documents, processors (in general) or presentation processors. The concern is about presentation processors, which is complicated by explicit potential for variations (available fonts, etc.). Some qualifications are needed of the form “all allowed variations being equal”;
In this exercise I was concerned with how a TTML1 document (not containing any TTML2 feature) would be rendered by a TTML2 presentation processor, in particular if the rendering would be an acceptable results according to TTML1. So this includes all variations allowed per TTML1.


  1.
  2.  Our editor came to a different conclusion (there are compatibility issues) with specific examples on a recent call, so we need to resolve that. Perhaps these are captured in the orange highlight (or should I say #FFA500 😊; and
From the 10/12 call, I can see:
- lineHeight. Covered in my document.
- displayAspectRatio, but it is a new TTML2 feature, I'm assuming it was meant to be pixelAspectRatio. Covered in my document.
Have I missed anything?

I'm concerned about:
- Pierre's words:
"basic things like lineHeight style inheritance."
I did not see the changes in TTML2 that would affect style inheritance. I may have missed it, so if anyone could point to those differences, it'd be very useful.

- Glenn's words:
"There are layers that affect every feature - it's not simple than just talking about
... individual style features."
I'd like to understand what this means. I understand that there are general aspects that affect every feature (like style resolution) or relationships between style features (like DAR vs. PAR) but I didn't see "layers that affect every feature" differently between TTML1 and TTML2.
Glenn, can you clarify? or give an example?



  1.
  2.  Without consensus of an explicit stated goal of presentation processor backwards compatibility, it doesn’t mean it won’t break before publication as a Rec or be unintentionally vague; But we can’t seem to bring ourselves to make such a statement for some reason despite an agreement in principle 2 meetings ago to do so (and my assignment to propose TTML2 spec language). But maybe this exercise will make everyone more comfortable doing that.
Hopefully that should help the discussion stay focused and avoid discussing for hours broad statements.

Regards,
Cyril

  1.

Regards,
               Mike

From: Nigel Megitt [mailto:nigel.megitt@bbc.co.uk<mailto:nigel.megitt@bbc.co.uk>]
Sent: Tuesday, October 17, 2017 10:00 AM
To: Cyril Concolato <cconcolato@netflix.com<mailto:cconcolato@netflix.com>>; public-tt@w3.org<mailto:public-tt@w3.org>
Subject: Re: TTML2/TTML1 Backwards compatibility analysis

Thanks Cyril,

I think it's very useful to focus on the concrete differences rather than the abstract – we may find that we can resolve them.

I've added this (practical compatibility issues between TTML1 and TTML2) to Thursday's agenda.

The URL in your email had an error in: the correct one should be https://docs.google.com/document/d/1Ri7RBBsbIK9SRxA1KsHRejXbYBuL4CRRrbmEZRbwZpg/edit?usp=sharing


In the meantime if everyone interested in this could look at the document and comment/edit it etc that would be very helpful.

Kind regards,

Nigel


From: Cyril Concolato <cconcolato@netflix.com<mailto:cconcolato@netflix.com>>
Date: Saturday, 14 October 2017 at 01:09
To: "public-tt@w3.org<mailto:public-tt@w3.org>" <public-tt@w3.org<mailto:public-tt@w3.org>>
Subject: TTML2/TTML1 Backwards compatibility analysis
Resent-From: <public-tt@w3.org<mailto:public-tt@w3.org>>
Resent-Date: Saturday, 14 October 2017 at 01:11

Hi all,

Following yesterday's call, I started an analysis of the possible backwards compatibility issues of TTML2 vs TTML1. The results are here:
https://docs.google.com/document/d/1Ri7RBBsbIK9SRxA1KsHRejXbYBuL4CRRrbmEZRbwZpg/edit?usp=sharing<https://docs..google.com/document/d/1Ri7RBBsbIK9SRxA1KsHRejXbYBuL4CRRrbmEZRbwZpg/edit?usp=sharing>

This is my analysis, and it might contain errors, oversights. If it is the case, feel free to comment on it.

With the current status, it looks to me that there is no real backwards compatibility issue, in the sense that a TTML2 processor would produce a result, when processing a TTML1 document, that would be acceptable with what the TTML1 spec indicates.

HTH,
Cyril





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